1. Field of the Invention
The present invention relates to a process for surface treatment of a hollow work having a hole communicating with the outside, particularly, a ring-shaped work such as a ring-shaped bonded magnet, and to a ring-shaped bonded magnet produced by the process. More particularly, the present invention relates to a surface treating process which comprises bringing a fine metal powder producing material into flowing contact with the surface of a work, thereby adhering a fine metal powder produced from the fine metal powder producing material to the surface of the work, and to a ring-shaped bonded magnet produced by the process and having a film layer made of the fine metal powder on the entire surface thereof.
2. Description of the Related Art
A rare earth metal-based permanent magnet such as an Rxe2x80x94Fexe2x80x94B based permanent magnet represented by an Ndxe2x80x94Fexe2x80x94B based permanent magnet is produced using a material which is rich in resources and inexpensive and has a high magnetic characteristic, as compared with an Smxe2x80x94Co based permanent magnet. Therefore, particularly, the Rxe2x80x94Fexe2x80x94B based permanent magnet is used in a variety of fields at present.
In recent years, in electronic industries and appliance industries, a reduction in size of parts is advancing, and in correspondence to this, a reduction in size and a complication in shape of the magnet itself are required.
From this viewpoint, attention is given to a bonded magnet which is produced using a magnetic powder and a resin binder as main components and which is easy to shape. Among others, a ring-shaped bonded magnet is utilized particularly in various small-sized motors such as a spindle motor, a servomotor incorporated in an actuator and the like.
The rare earth metal-based permanent magnet contains a rare earth metal R which is liable to be corroded by oxidation in the atmosphere. Therefore, when the rare earth metal-based permanent magnet is used without being subjected to a surface treatment, the corrosion advances from the surface of the magnet under the influence of a small amount of an acid, an alkali or water to generate a rust in the magnet, thereby bringing about the deterioration and dispersion of the magnetic characteristic. Further, when the magnet having a rust generated therein is incorporated in a magnetic circuit, it is feared that the rust is scattered to pollute the surrounding parts.
To solve this problem, an attempt has been made to form a plated film as a corrosion-resistant film on the surface of the magnet. However, when the bonded magnet is subjected directly to an electroplating treatment, a uniform and dense plated film cannot be formed, because the magnetic powder particles insulated by the resinous binder forming the surface of the magnet and the resin portion between the magnetic powder particles are lower in electric conductivity. As a result, pinholes (non-plated portions) may be produced to bring about a rust in some cases.
Therefore, processes which involve providing an electric conductivity to the entire surface of a bonded magnet and subjecting the bonded magnet to an electroplating treatment, have been already proposed in Japanese Patent Application Laid-open Nos. 5-302176, 7-161516, 11-3811 and the like.
These processes proposed in the above Patents are intended to provide the electric conductivity to the entire surface of the magnet by adhering a metal powder to the entire surface of the magnet by utilizing the tackiness of a third component such as a resin and a coupling agent. In such processes, however, it is difficult to adhere the metal powder uniformly to the inner surface of a ring-shaped bonded magnet and among others, a ring-shaped bonded magnet having a large L/D value (wherein L represents a length of the magnet in a direction of a center axis of the magnet, and D represents an inside diameter of the magnet) to form an electrically conductive layer. This is because as the larger the L/D value, the more both of the metal powder and the third component such as the resin are sufficiently not spread into the through-hole in the magnet.
Further, in these processes, an increase in cost is brought about, because the third component is required, and moreover, it is difficult to uniformly form the electrically conductive layer on the entire surface of the magnet. As a result, it is difficult to carry out the surface treatment at a high dimensional accuracy. Additionally, a step of curing an uncured resin is required, resulting in a complicated producing process. Furthermore, when media such as steel balls are employed as a metal powder adhering means, it is feared that cracking or chipping of the bonded magnet may be brought about.
Accordingly, it is an object of the present invention to provide a process for surface treatment of a ring-shaped bonded magnet, wherein an electric conductivity can be provided to the entire surfaces of the magnet, i.e., not only to the outer surface (including end faces and so on) but also to the inner surface of the magnet without use of a third component such as a resin and a coupling agent, and a film having an excellent corrosion resistance can be formed on the surface of the magnet at a high thickness accuracy by an electroplating treatment or the like.
The present inventors have made various studies with the forgoing in view and as a result, they have found that when a fine metal powder producing material is brought into flowing contact with the surface of a hollow work having a hole communicating with the outside in a treating vessel, a fine metal powder is produced from the fine metal powder producing material and brought in a flowing state into contact with not only the outer surface but also the inner surface of the work, whereby it is adhered firmly and at a high density to the entire surface of the work.
The present invention has been accomplished based on such knowledge, and to achieve the above object, according to a first aspect and feature of the present invention, there is provided a process for surface treatment of a hollow work having a hole communicating with the outside, comprising the steps of placing the work and a fine metal powder producing material into a treating vessel, and bringing the fine metal powder producing material into flowing contact with the surface of the work in the treating vessel, thereby adhering a fine metal powder produced from the fine metal powder producing material to the surface of the work.
According to a second aspect and feature of the present invention, in addition to the first feature, the flowing contact of the fine metal powder producing material with the surface of the hollow work is achieved by rotating the treating vessel.
According to a third aspect and feature of the present invention, in addition to the second feature, the treating vessel is cylindrical in shape, and the flowing contact of the fine metal powder producing material with the surface of the hollow work is achieved by rotating the cylindrical treating vessel about its center axis.
According to a fourth aspect and feature of the present invention, in addition to the first feature, the hollow work having the hole communicating with the outside is a ring-shaped work.
According to a fifth aspect and feature of the present invention, in addition to the fourth feature, the ring-shaped work is placed into the cylindrical treating vessel, so that its center axis is parallel to a center axis of the cylindrical treating vessel, and the flowing contact of the fine metal powder producing material with the surface of the ring-shaped work is achieved by rotating the cylindrical treating vessel about its center axis.
According to a sixth aspect and feature of the present invention, in addition to the fifth feature, a rod-shaped member is inserted through and disposed in the through-hole in the ring-shaped work, so that it is parallel to the center axis of the ring-shaped work.
According to a seventh aspect and feature of the present invention, in addition to the fourth feature, the ring-shaped work is a ring-shaped rare earth metal-based permanent magnet.
According to an eighth aspect and feature of the present invention, in addition to the seventh feature, the ring-shaped rare earth metal-based permanent magnet is a ring-shaped bonded magnet.
According to a ninth aspect and feature of the present invention, in addition to the first feature, the fine metal powder producing material is a material for producing a fine powder of at least one metal selected from the group consisting of Cu, Fe, Ni, Co, Cr, Sn, Zn, Pb, Cd, In, Au, Ag and Al.
According to a tenth aspect and feature of the present invention, in addition to the ninth feature, the fine metal powder producing material is a fine Cu powder producing material.
According to an eleventh aspect and feature of the present invention, there is provided a ring-shaped bonded magnet having a film layer made of a fine metal powder on the entire surface thereof, which is produced by a surface treating process according to the first feature.
According to a twelfth aspect and feature of the present invention, in addition to the eleventh feature, the ring-shaped bonded magnet having the film layer made of the fine metal powder on the entire surface thereof has an L/D value equal to or larger than 1, wherein L represents a length of the magnet in a direction of a center axis of the magnet, and D represents an inside diameter of the magnet.
According to a thirteenth aspect and feature of the present invention, there is provided a ring-shaped bonded magnet having a plated film, which is produced by subjecting a ring-shaped bonded magnet having a film layer made of a fine metal powder on the entire surface thereof according to the eleventh or twelfth feature to an electroplating treatment.
With the surface treatment process according to the present invention, the fine metal powder produced from the fine metal powder producing material is adhered firmly and at a high density to the entire surface of the hollow work having the hole communicating with the outside, i.e., not only to the outer surface but also to the inner surface of the work, by bringing the fine metal powder producing material into flowing contact with the surface of the hollow work having the hole communicating with the outside. Therefore, with the ring-shaped rare earth metal-based permanent magnet, an electric conductivity can be provided to the entire surface of the magnet without provision of a resin layer on the surface of the magnet. Thus, a film having an excellent corrosion resistance can be formed at a high thickness accuracy by an electroplating treatment.